1. Field of the Invention
This invention relates to multilayered structures useful for forming capacitors. More particularly, the invention pertains to polymeric capacitors, which may be embedded within printed circuit boards or other microelectronic devices. The capacitor comprises a pair of parallel conductive foils separated by a pair of dielectric layers. The dielectric layers are separated by a heat resistant film layer such that the capacitor exhibits excellent short circuit resistance.
2. Description of the Related Art
The operational speed of computers has rapidly increased as a result of improvements in the production of microelectronic devices. As the clock speed of the personal computer exceeds GHz levels, increased device signal transmission speed is required. This is particularly true as local area networks (LAN) are more widely used. In-house LAN systems have become more common due to the need for management of multiple connected computers. This is because LANs provide a very large memory, which can control a vast amount of information. High-speed operational performance also enables several computers to access data simultaneously. As the circuitry design of central processing units (CPU) seeks to achieve these system requirements, the performance of integrated circuits becomes ever more important. The signal transmission function in the server must be fast and have few malfunctions. However, the circuitry design of printed circuit boards, which mount these integrated circuits, is also very important.
Capacitors are common elements on printed circuit boards and other microelectronic devices. Capacitors are used to steady the operational power supply of such devices. A capacitor is a device used for introducing capacitance into a circuit, and functions primarily to store electrical energy, block the flow of direct current, or permit the flow of alternating current. They comprise a dielectric material sandwiched between two electrically conductive metal layers, such as copper foils. In general, the dielectric material is coupled to the electrically conductive metal layers via an adhesive layer, by lamination, or by vapor deposition.
Heretofore, capacitors arranged on the surface of printed circuit boards have been common. However, more recently, capacitors are formed of a thin, double-sided copper clad laminate within multilayered circuit board layers thus producing excellent characteristics. Of these options, it has been preferred to form printed circuit boards having embedded capacitors to maximize the surface area of the circuit board for other purposes. In order to achieve increased signal transmission speed, printed circuit board manufacturers generally form printed circuit boards within such a multilayer structure. The capacitance of a capacitor depends primarily on the shape and size of the capacitor layers and the dielectric constant of the insulating material. There are various known types of dielectric materials known in the art. For example, the dielectric material may be a gas, such as air, a vacuum, a liquid, a solid or a combination thereof. Each material has its own particular properties.
In forming capacitors for use in printed circuit boards, a dielectric material such as a glass reinforced polymer matrix has been used. However, the performance of capacitors of this type has been limited by factors such as limited minimum thickness of the dielectric material, which detracts from the flexibility of the capacitor, the attainable capacitance, the effect of bond enhancers on the metal foils, low dielectric constant and poor dielectric strength.
It is desirable to form a capacitor for circuit boards having a high dielectric constant and an extremely thin layer of dielectric material, thus increasing the capacitance and flexibility of the capacitor. To optimize the performance of a capacitor, it is important that the dielectric material employed have good material properties, exhibiting such qualities as superior adhesion, high dielectric strength and good flexibility. However, common problems frequently associated with extremely thin dielectric layers are the formation of microscopic voids or other structural defects and inclusion of foreign material. These lead to electrical shorts. For example, U.S. Pat. Nos. 5,155,655 and 5,161,086 describe a method for forming a capacitor wherein a single sheet of a dielectric material is laminated together with two conductive foils. Dielectric layers of this type are highly vulnerable to the formation of voids, as well as the inclusion of foreign material, and are time consuming to detect and remedy.
The present invention provides a capacitor that solves the problems of the prior art. The capacitors of the present invention comprise a pair of conductive foils, a pair of thin dielectric layers, with one dielectric layer on a surface of each of the foils. The two conductive foils are adhered together such that the dielectric layers are attached to one another via an intermediate heat resistant film layer.
By using two thin dielectric layers, any structural voids present in one of the individual dielectric is covered when the other dielectric is adjoined. The likelihood that each of the two dielectric layers has a void congruent with a void on the other dielectric layer is extremely remote. This increases the reliability and physical strength of the capacitor and eliminates a source of manufacturing flaws. Also, the thin dielectric layers allow for higher capacitance, greater heat conductivity and greater flexibility of the capacitor. The intermediate heat resistant film layer deters the formation of electrical shorts between the electrically conductive foils. Together, these layers offer a significant improvement in performance over prior art capacitors and printed circuit boards.
The invention provides a multilayered construction suitable for forming capacitors which comprises sequentially attached layers comprising: a first electrically conductive layer, a first thermosetting polymer layer, a heat resistant film layer, a second thermosetting polymer layer and a second electrically conductive layer.
The invention also provides a capacitor which comprises sequentially attached layers comprising: a first electrically conductive layer, a first thermosetting polymer layer, a heat resistant film layer, a second thermosetting polymer layer and a second electrically conductive layer.
The invention still further provides a method of forming a multilayered construction for forming capacitors, which comprises sequentially attaching layers comprising a first electrically conductive layer, a first thermosetting polymer layer, a heat resistant film layer, a second thermosetting polymer layer and a second electrically conductive layer.
The invention still further provides a method of forming a multilayered construction for forming capacitors which comprises attaching a first thermosetting polymer layer to a surface of a first electrically conductive layer; attaching a second thermosetting polymer layer to surface of a second electrically conductive layer; and then laminating the first and second thermosetting polymer layers to opposite surfaces of a heat resistant film layer.
The invention also provides a method of forming a multilayered construction for forming capacitors which comprises attaching a first thermosetting polymer layer to a first surface of a heat resistant film layer; attaching a second thermosetting polymer layer to a second opposite surface of the heat resistant film layer; applying a first electrically conductive layer to the first thermosetting polymer layer and applying a second electrically conductive layer to the second thermosetting polymer layer.